The present invention relates generally to the field of regulated automotive vehicle alternator battery charging systems in which an alternator supplies a rectified charging signal to maintain the battery in a charged condition by sensing the battery voltage and utilizing a voltage regulator in a closed feedback system to control the alternator field coil excitation current to control the charging output of the alternator. Typically, transistorized alternator voltage regulators are utilized to control the alternator field coil excitation in response to the sensed battery voltage. These systems act to maintain the battery voltage substantially constant, generally between high and low voltage limits, and thereby maintain the battery in a charged condition.
Some prior charging systems provide a continuous proportional DC field coil excitation current in response to the sensed battery voltage, but these systems suffer from excessive power dissipation in the transistor control device that provides the continuous field coil excitation. More typically, present day transistor voltage regulators are operative in an on/off mode wherein a transistor device is utilized to alternately stepwise excite the alternator field coil between full and zero excitation, wherein both the rate of this alternate stepwise excitation and the duration of the periods of excitation and de-energization of the field coil vary in accordance with engine speed and the resistive load placed upon the battery. An example of such a battery charging system using an on/off regulator in U.S. Pat. No. 3,273,049 assigned to the same assignee as the present invention. This type of battery charging system has the advantage of minimizing the power dissipation requirements required for the transistor control device that controls the field coil excitation. However, these types of charging systems are subject to creating low frequency ripple voltages across the battery due to the low frequency, intermittent battery charging operation of the alternator. This results in having automotive accessories which are run off of the battery, such as the automobile headlights and instrument indicator lights, suffer from low frequency battery voltage variations which in the case of headlights will cause an annoying flickering of the headlights. This phenomena becomes extremely pronounced at low engine speeds since under those conditions it may take a significant amount of time for the alternator to charge up the battery once the battery voltage has fallen below a minimum threshold. In addition, battery charging systems of the above type also suffer from overshoot problems wherein in order to provide the desired switching action of the alternator field current, maximum and minimum switching levels have to be built into the voltage regulator sensing circuit. Thus when a low battery voltage is sensed, the alternator must charge the battery to a value above its actual desired charge value and substantially above the sensed minimum battery voltage threshold level in order to terminate the field coil excitation. If the on/off field excitation levels are too close, then power dissipation for the control device becomes a problem, and if they are too widely spaced then at low engine speeds the headlights will flicker.